The present invention generally relates to rewritable optical recording medium for use in the recording of information, and more particularly, to an external memory apparatus for a computer, a video and/or audio recording apparatus, a memory apparatus for game machines or the like, or a multimedia system in which these units are integrated.
In recent years, a magneto-optical disk, which is a rewritable optical recording medium, has been put into practical use as an optical recording medium capable of reading, writing, and erasing operations.
Conventionally guide grooves for guiding laser beams for recording/erasing are provided in generally used optical disks so that information to be written is adapted to be written in the above described guide grooves or among the guide grooves.
In recent years, data has become wider and, further, the demand for higher density has increased because of popularization of optical disks. A system of writing on both a guide groove (in-groove) and between-guide-grooves (on-land) for track guidance has been invented as one of the solutions. The technical contents thereof are disclosed in U.S. Pat. No. 4,423,502.
This method can realize a track pitch of 1/2 without reduction in tracking error signals, as the in-groove portions and the on-land portions are used as recording tracks. In a conventional system, an effect of writing signals on both the groove concave portion and the groove convex portion cannot be obtained, because cross-talk from adjacent tracks increases when a track pitch is made smaller than a beam diameter.
Concretely describing, a track pitch is limited to 1.4 .mu.m under the influence of the cross-talk from adjacent tracks when a laser wavelength of a reading optical head is 830 nm, and 0.55 in NA (Numerical Aperture). In the optical head, it is not necessary to use both the in-groove portions and the on-land portions, because sufficient tracking error signals can be obtained if a tracking pitch of 1.4 .mu.m is adapted to be written on on-land portions or in-groove portions only.
Generally, in the optical magneto-optical disk, a written track is divided into a plurality of sectors, and signals constituting address information are recorded in a disk manufacturing stage as pits on the head of each sector. Information about specifications of a disk, appliance setting conditions about recording/reading/erasing operations, signals necessary for servo-adjustment and so on are likewise written at a disk manufacturing stage as a signal in the form of pits.
According to U.S. Pat. No. 4,423,502 disclosed here, portions written as pits are adapted to form a pitch of a constant depth independently of the in-groove portions and the on-land portions as shown in FIG. 5.
A cutting operation for regulating the depth of a groove and a cutting operation for regulating the depth of a pit have to be effected separately during stamping to form such a substrate. Normally there are a method of regulating the thickness with which photoresist is applied on a master substrate, and a method of controlling exposure strength when it is laser-exposed so as to regulate the depth of the groove or pit during cutting. In forming a groove or a pit rectangular in section, the method of regulating the depth of groove or pit with photoresist thickness can be carried out stably and easily. But this method has a problem in that the depth of the groove or pit is determined in one way by the photoresist thickness so that it is extremely difficult to form pits with the same depth independently of the presence of a groove as shown in FIG. 5.
In order to solve the problem, a method is known for providing pits only in portions not connected with the groove without providing pits in the groove portion. In a method shown in FIG. 6 (Japanese Patent Laid-Open Publication No. 2-156423), the pit information of address and so on is provided within the non-groove area on the extended line of the groove and the extended line of the groove interval. Since the groove and the pit exist in different positions, the depth can be determined in one way by the photoresist thickness when the depth is made the same, thereby enabling the manufacturing operation to be effected more easily.
A switching operation between a scanning operation of the in-groove portions of the guide groove with optical beams and a scanning operation of the on-land portions with optical beams is achieved by positive and negative inverted rotation of two inputs of a differential amplifier for obtaining a tracking error signal as disclosed in Japanese Patent Laid-Open Publication No. 57-50330. That is, it is a method of switching a polarity of a tracking error signal.
If the polarity of the tracking error signal is set up so as to scan the optical beams on the pits in FIG. 6, it can scan the interior of the groove by the same polarity, but cannot scan on the on-land portion.
When a tracking has been set as in the scanning operation of the on-land with the optical beams, in a disk as shown in FIG. 6, diffraction lights to be reflected from a pit portion become of a polarity opposite to an on-land portion in a pit portion provided on the extension line of the on-land, and optical beams are adapted to scan between pit strings, so that reading of pit information cannot be correctly effected.